The negative ion chemical ionization (NCI) mass spectra of the pentafluoroproprionyl (PFP) and heptafluorobutyryl (HFB) derivatives of 2-aminophenol contain a base ion which is generated by a mechanism involving the a loss of 44 amu, and ions reflecting this loss are also observed in the NCI spectra of the PFP or HFB derivatives of other ortho hydroxyanilines, including 3-hydroxybenzidine and the n-propyl esters of 2-hydroxyanilinic acid and 2-aminotyrosine. A corresponding ion is not observed in the NCI spectra of the trifluoroacetyl (TFA) derivative of 2-aminophenol or in the spectra of the PFP or HFB derivatives of meta or para isomers of hydroxyaniline or of 2-ethylaminophenol. Examination of the mechanism of this loss by tandem mass spectrometry, linked scanning, and high resolution mass spectrometry revealed that it reflects elimination of CO2 and indicate that the fragmentation pathway involves an initial loss of HF, in which the hydrogen atom is derived from the deriv atized amide nitrogen, followed by formation of a ring intermediate from which CO2 is subsequently eliminated to yield another ring structure. An ion generated by loss of 44 amu is also observed in the NCI spectra of HFB or PFP derivatives of catechols, including L-dopa and catechol itself, and the mechanism of this loss may also involve formation of a ring intermediate from which CO2 is subsequently eliminated. The occurrence of this loss of CO2 can be used to achieve distinction among regio-isomers of hydoxyanilines and to distinguish a catechol from a resorcinol or hydroquinone nucleus.